1.Field of the Invention
The present invention relates to a display device and a driving method thereof, and more particularly, to an electro-luminescence display device and a driving method thereof.
2.Description of the Related Art
Cathode ray tubes (CRTs) are heavy and bulky as display devices. To solve these disadvantages of the CRTs, flat display devices have been developed. Examples of flat display devices include a liquid crystal display device (LCD), a field emission display device (FED), a plasma display device (PDP), and an electro-luminescence (EL) display device. The EL display device is a self-luminous device that emits light from a fluorescent material in which recombination of electrons and holes occurs. The EL display device can be classified into an inorganic EL display device and an organic EL display device, depending on the fluorescent materials and structures used. Unlike the LCD, the organic EL display device does not require a separate light source. Therefore, the organic EL display (hereinafter, referred to as an OLED) device has a rapid response time comparable to that of the CRT.
FIG. 1 is a cross-sectional view of an EL cell in a related art organic electro-luminescence display panel. More specifically, FIG. 1 is a cross-sectional view of an organic EL structure for explaining the light emitting structure of the OLED device. Referring to FIG. 1, the OLED device includes an electron injection layer 4, an electron transport layer 6, an organic emission layer 8, a hole transport layer 10, and a hole injection layer 12, which are sequentially stacked between a cathode 2 and an anode 14. When a predetermined voltage is applied between a transparent electrode as the anode 14 and a metal layer as the cathode 2, electrons from the cathode 2 move toward the emission layer 8 through the electron injection layer 4 and the electron transport layer 6. Also, holes from the anode 14 move toward the organic emission layer 8 through the hole injection layer 12 and the hole transport layer 10. The electrons from the electron transport layer 6 and the holes from the hole transport layer 10 recombine in the organic emission layer 8, thereby generating light. Then, the light is emitted to the outside through the transparent electrode anode 14 of the transparent electrode.
FIG. 2 is a circuit diagram of a related art organic EL display device. Referring to FIG. 2, the related art OLED device includes an organic EL display panel 16, a scan driver integrated circuit (scan D-IC) 18, a data driver integrated circuit (data D-IC) 20, and a timing controller 26. The OLED panel 16 includes subpixels 22 formed on regions defined by a plurality of scan lines SL1 to SLn and a plurality of data lines DL1 to DLm that cross each other. The scan D-IC 18 drives the scan lines SL1 to SLn, and the data D-IC 20 drives data lines DL1 to DLm. Additionally, the timing controller 26 controls the driving timing of the data D-IC 20 and the scan D-IC 18. Each of the subpixels 22 includes a power source VDD, a ground source GND, an OLED cell connected between the power source VDD and the ground source GND, and an OLED driver circuit 24 for driving the OLED cell in response to driving signals supplied from the data line DL and the scan line SL. One pixel is constructed with red (R), green (G) and blue (B) subpixels that are horizontally arranged adjacent to one another.
The OLED driver circuit 24 includes: a driving thin film transistor (hereinafter, referred to as a TFT) DT connected between the power source VDD and the OLED device: a first switching TFT T1 connected to the scan line SL and the data line DL; a second switching TFT T2 connected to the first switching TFT T1 and driving TFT DT; a converting TFT MT connected between the power source VDD and a node of the first switching TFT T1 and the second switching TFT T2, the converting TFT MT forming a current mirror circuit together with the driving TFT DT to convert a current into a voltage; and a storage capacitor Cst connected between the power source VDD and a node of gates of the driving TFT DT and the converting TFT MT. The driving TFT DT, the converting TFT MT, the first switching TFT T1 and the second switching T2 are formed of p-type metal-oxide semiconductor field effect transistors (MOSFETs).
The driving TFT DT has a gate connected to the gate of the converting TFT MT, a source connected to the power source VDD, and a drain connected to the OLED device. The converting TFT MT has a source connected to the power source VDD, and a drain connected to a drain of the first switching TFT T1 and a source of the second switching TFT T2. The first switching TFT T1 has a source connected to the data line DL, and a drain connected to a source of the second switching TFT T2. The second switching TFT T2 has a drain connected to the gates of the driving TFT DT and the converting TFT MT and the storage capacitor Cst.
The first and second switching TFTs T1 and T2 have gates connected to the scan line SL. The converting TFT MT and the driving TFT DT form a current mirror circuit because they were formed to have the same electrical characteristics. If the converting TFT MT and the driving TFT DT are the same, the amount of current flowing through the converting TFT MT will be identical to the amount of current flowing through the driving TFT DT.
The timing controller 26 generates a data control signal for controlling the data D-IC 20 and a scan control signal for controlling the scan D-IC 18 by using synchronization signals supplied from an external system, such as a graphic card. Also, the timing controller 26 supplies the data D-IC 20 with a video data supplied from the external system. The scan D-IC 18 generates scan signals in response to the scan control signal supplied from the timing controller 26.
FIG. 3 is a waveform of scan signals supplied to scan lines of FIG. 2. As illustrated in FIG. 3, the scan signals are supplied to the scan lines SL1 to SLn, so that the scan lines SL1 to SLn are sequentially driven. The data D-IC 20 supplies the data lines DL1 to DLm with data signals having a current level or a pulse width responsive to the video data at every horizontal period 1H according to the data control signal supplied from the timing controller 26. At this point, the data D-IC 20 has m number of output channels 21 that match with the data lines DL1 to DLm in 1:1 correspondence.
The data D-IC 20 supplies each of the subpixels 22 with the data signal having a current level or a pulse width proportional to an input data. Each of the subpixels 22 emits light in proportion to an amount of the current supplied from the data line DL. Because a pixel is constructed of red (R), green (G) and blue (B) subpixels horizontally arranged, three data lines and one scan line are required to drive the related art pixel.
In the related art OLED device, the scan D-IC 18 has outputs that are matched 1:1 with the scan lines SL1 to SLn in a row direction of the organic EL display panel 16, and the data D-IC 20 has channels 21 matched 1:1 with the data lines DL1 to DLm in a column direction of the organic EL display panel 16. Since the output channels 21 of the data D-IC 20 are matched 1:1 with the data lines DL1 to DLm, as many output channels 21 of the data D-IC 20 are required as there are data lines DL1 to DLm. Consequently, the related art organic EL display device has drawbacks in that the price of the data D-IC 20 increases as the number of the output channels 21 of the data D-IC 20 increases. In turn, the number of output channels 21 increases as the size of the OLED panel 16 increases.